Resistor for power distribution circuits

ABSTRACT

A surge suppressing resistor is enclosed in an insulating housing. The resistor is wound on a grooved epoxy core and is coated with epoxy. Terminal portions are locked in either end of the core.

[ Mar. 4, 1975 [56] References Cited UNITED STATES PATENTS 2,114,458 4/1938 338/269 2,280,367 4/1942 Barton............................ 338/270 X FOREIGN PATENTS OR APPLICATIONS 570,761 2/l933 Germany......................,.....338/237 Primary Examiner-E A. Goldberg Attorney, Agent, or FirmRobert C. Jones [57] ABSTRACT A surge suppressing resistor is enclosed in an insulating housing. The resistor is wound on a grooved epoxy core and is coated with epoxy. Terminal portions are locked in either end of the core.

7 Claims, 5 Drawing Figures CIRCUITS [75] Inventor: Norman W. Stunkard, Lake Oswego, Oreg.

Allis-Chalmers Corporation, Milwaukee, Wis.

May 6, 1974 Appl. No.: 467,553

338/231, 338/234, 338/237, 338/265, 338/270, 338/274, 338/275 Int. HOIc 1/00 Field of Search 338/232, 234, 231, 262, 338/263, 264, 265,269, 270, 273, 274, 275, 303, 237, 324

United States Patent Stunkard 5 RESISTOR FOR POWER DISTRIBUTION [73] Assignee:

[22] Filed:

SUMMARY OF THE INVENTION This invention relates generally to electrical resistors and is directed particularly to the method of manufacturing which reduces the cost of manufacture and increases the reliability of the resistor.

The electrical utility industry has used shunting resistors for many years. However, with the advent of 345 KV and 500 KV transmission services, the resistors utilized for the purpose of suppressing overvoltages which might be caused by switching surges resulted in some extremely bad experiences. Some of the problems experienced with the resistors used at the voltages aforementioned were subject to internal turn-to-turn failures, external flashovers and open circuits which could cause a violent rupture due to a breakdown of the insulating oil into hydrogen gas. These failures were compounded by the limited thermal capacity of some of the resistor devices. In addition to the failures experienced, the relatively high cost of the resistors was a serious objection which in todays economy requires improvement.

The surge suppression resistor of the present invention has been designed to correct the earlier encountered problems. The resistor of the present invention can be used in applications that can be evaluated without the need of a system study. For example, they would be suitable for dropping magnetized current on 345 and 500 KV transformers with minimal overvoltage. They also may be used to energize these same unloaded transformers.

With the ohmic resistance and period of time in the circuit, the total energy input can be calculated. This value is then compared to the maximum recommended input of 42,000 joules (watts-seconds) in any one resistor module. In this type of application, the frequency of operation should be no more than twice per hour.

Another straightforward application to which the present resistor is particularly well adapted is switching bus charging current.

As herein disclosed, the resistor is of the axial type with the resistor wire wound in a spiral around the preformed casted core. The wire wound core is cast solid in a suitable dielectric epoxy which fills the spaces between adjacent windings and completely encapsulates the core including the resistor wire. The resistor wire is thus supported in a predetermined spaced apart coil format and also protected by an insulator layer. The encapsulated resistor is inserted into a tubular porcelain housing, and the housing is filled with a suitable insulation such as sulfur hexaflouride (SE6) gas, oil or a solid epoxy.

It is, therefore, an object of the invention to provide a resistor of great efficiency and which produces a low cost method of manufacturing.

Another object of the invention is to provide a resistor which is manufactured from a dielectric resin by means of a casting process.

Other objects and advantages will be apparent from the following description.

DRAWINGS FIG. I is a view of a cast resistor core showing the spiral grooving integrally cast in the peripheral surface of the core in which the resistor wire is wound;

FIG. 2 is a view of the resistor core with the resistor wire wound up thereon with the ends of the wires being electrically connected to the end plugs;

FIG. 3 is a view showing the wire-wound resistor core of FIG. 2 encapsulated in the dielectric epoxy;

FIG. 4 is an end view of the encapsulated resistor of FIG. 3 showing the symmetrical relationship of the parts and with an end plug therein; and

FIG. 5 is a showing of an assembled resistor assembly with parts broken away to show the interior relation ship of the various components.

DESCRIPTION OF THE INVENTION Referring to FIG. 1, there is shown a resistor core 11 which was cast in a suitable mold with a suitable resin to which a hardener is added. The resin is liquified and the hardener is added thereto and mixed thoroughly with the resin. The prepared epoxy, upon being poured into the mold, takes the form of the interior surface of the mold (not shown). To this end, the mold (not shown) is formed with a male spiral embossing so that the cast epoxy, when hardened, forms a solid core that presents a spiral groove throughout its length. At each end of the mold (not shown) end plugs 14 and 15 are supported so that the end plugs are cast in axial alignment with the poured core 11 as depicted in FIG. 1. To this end the end plugs are each formed with a peripheral groove 16 and 17 thereby presenting a pair of radially extending flanges 18 and 19 and 21 and 22, respectively. Thus, the epoxy, when poured into the mold (not shown), will seep into the radial grooves 16 and 17 to bind the end plates in fixed axial relationship to the core 11. In the casting process, the mold is preheated so as to minimize air entrapment in the poured epoxy. It has been found that in pouring the epoxy into the preheated mold a successful pour will result if the pour is of an even flow. This procedure will minimize turbulence and avoid air entrapment in the finished core 11. It has been found satisfactory to preheat the mold to a temperature of F. After the pouring has been accomplished, the mold is set aside, outside of the oven, for approximately two hours to dissipate exotherm. With the resistor core 11 cured, the cast resistor core 11 is sandblasted to clean and roughen the external surface thereof for subsequent bonding to the parent encapsulating epoxy. Cooled resistor wire 23 which, in the present instance, is a nichrome wire, is wound on the core 11 in the spiral groove 24 which is cast into the core 11. With the resistor wire 23 wound on the cast core 11, the core and wire assembly 26 is preheated again to at least 125F. in an encapsulating mold (not shown). The encapsulating mold is also preheated to at least 125F. and is partially filled with the epoxy mix, and the core and wire assembly 26 is placed in the preheated encapsulating mold and allowed to settle by its own weight to the bottom of the mold. This is done slowly to ensure that air is forced out of the inside diameter of the coil resistor wire 23. The preheating also helps to expell air. The encapsulating mold, with the core assembly 26 therein, is cured at approximately F. with the cure occurring from the bottom up to eliminate voids due to a lack of backfill during gelatin shrink. After the encapsulated resistor unit 31 has set up, it may be removed from the mold, as shown in FIG. 3, and given final cure for approximately four hours in an oven at 300F. When the encapsulated resistor unit 31 is removed from the curing oven, it is suspended in free air for cooling so that it cools uniformly and is not subject to distortion forces.

As previously mentioned, the end plugs 14 and 15 are cast integrally with the resistor core 11 being embedded therein by means of the radial grooves 16 and 17 that are formed in each of the end plugs themselves. However, to insure that the end plugs 14 and 15 do not rotate relative to the cast core 11, the surfaces 36, 37 of the end plugs l4, 15, respectively, which face inwardly are provided with a pair of holes 38, 39 and 41, 42, respectively, which are formed from the inner surface of the plugs and do not extend clear through. These holes 38, 39 and 41, 42 provide a space in which the liquid epoxy enters to form protrusions 43, 44 and 46, 47 which have intimate engagement in the holes and resist the stress which tends to rotate the end plugs.

As shown in FIG. 3, the end plugs 14 and 15 are each provided with axial threaded openings 51 and 52 which are adapted to receive threaded contact studs 53 and 54, respectively. The end plugs 14 and 15 are also provided with a smaller threaded opening that receives threaded screws 56 and 57. The screws 56 and 57 pass through the eye of wire end terminals 58 and 59 which are silver soldered on the respective ends of the resistor wire 23. Thus, end-to-end electrical continuity is established from the contact stud 53 to the contact stud 54.

The encapsulated resistor unit 31 is enclosed within a porcelain enclosure or housing 61 that is glazed over its entire diameter and length. Aluminum end caps 62 and 63 are attached to the porcelain housing 61 by semiresilient epoxy which allows for the difference in the coefficient of expansion between the aluminum end caps and the porcelain. The encapsulated resistor unit 31 is inserted within the housing 61 and the ends sealed by means of plates 66 and 67 that are screw fastened to the end caps. The end plates 66 and 67 are provided with blind bores 68 and 69 that receive the ends of the contact studs 53 and 54 adjacent thereto. Both plates are screw fastened to the end caps with circular gaskets 71 and 72 providing a tight seal between the sealing plate and the end caps. As shown in FIG. 5, the sealing plates 66 and 67 are provided with axial hubs 77 and 78 in which the blind bores 68 and 69, respectively, are formed. Springs 73 and 74 disposed within the blind bores 68 and 69 are adapted to engage the adjacent axial ends of the contact studs 53 and 54. Thus, both end plates present a resilient spring member which serves to center the encapsulated resistor component within the porcelain housing.

The space 76 between the inner surface of the porcelain housing and the encapsulated resistor unit 31 is filled with a dielectric medium such as sulfur hexafluoride (SP under a suitable pressure or with oil of suitable quality or by a dielectric epoxy.

From the foregoing description a highly successful axial type resistor is provided and a method for manufacturing the resistor to provide a low cost unit has been described in considerable detail so that those skilled in the art may take full advantage of the discovery herein described.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as fol a metallic end plug intimately secured in each end face of said epoxy core;

a resistor wire wound on said core in the spiral groove therein;

means to electrically connect the ends of said resistor wire to an adjacent metallic end plug;

a covering of epoxy material encapsulating said wire wound epoxy core, said covering having intimate bonding contact with the epoxy material of said core to form a unitary encapsulated resistor unit; and,

electrical contact means extending outwardly from each of said metallic'end plates to provide connections to electrical contacts.

2. An electrical resistor unit according to claim 1 wherein said metallic end plugs are each formed with a circumferential groove, said plugs being positioned so as to be molded in said core in a manner that the epoxy material of said core completely fills the circumferential grooves of said end plates;

said resistor wire is in the form of wire coil, with each turn being spaced equidistant apart; and,

said epoxy covering being intimately disperse through the turns of said resistor wire coil.

3. An electrical resistor according to claim 2 wherein said end plugs are provided with means for effecting an intimate interlock between said epoxy material of said core and said end plugs;

whereby rotation of said end plugs relative to said core is resisted by said interlock means.

4. An electrical resistor unit according to claim 3 wherein said means for effecting an intimate interlock between said end plugs and said epoxy core is at least one blind opening offset from the axes of said end plugs, and formed in the face of said plugs that face inwardly towards each other;

whereby the epoxy material when said core is cast will enter into said openings to effect an interlocking engagement between said epoxy material of said core and said metallic end plugs to resist any tendency of said metallic plugs to rotate relative to said core.

5. An electrical resistor;

a solid cylindrical body defining a core formed of an epoxy material and having a spiral groove extending from one end to the other formed in the cylindrical surface thereof;

a metallic end plug molded integrally with said core in each axial end of said core;

electrical contact studs extending axially outwardly from said metallic end plugs;

a coiled resistor wire wound on said core in the spiral groove thereof and extending from one end of said core to the other;

means electrically connecting each end of said resistor coil wire to an adjacent metallic end plug to provide electrical continuity from one metallic end plug to the other;

an epoxy covering intimately bonded to the epoxy material of said core to form an encapsulated resistor assembly, said epoxy covering being dispersed through the coils of said resistor wire to completely enwrap the wire in the epoxy material;

a porcelain tubular housing having an internal diameter larger than the external diameter of said encapsulated resistor assembly to receive said resistor assembly therein;

3 ,8 69,69 1 5 6 metallic end closures engaged on each end of said 6. An electrical resistor according to claim 5 wherein porcelain housing, said end closures being sealed to the void within said housing is filled with urethane of the ends of said housing, said enclosures having soft duro'meter. electrical contact with an adjacent axially extend- 7. An electrical resistor according to claim 5 wherein ing contact stud; and, 5 the void within said housing is filled with an insulating sulfur hexafluoride insulation filling the void within oil.

said housing. 

1. An electrical resistor; a cylindrical core of an epoxy material, said core having a spiral groove formed in its cylindrical surface and extending from end-to-end thereof; a metallic end plug intimately secured in each end face of said epoxy core; a resistor wire wound on said core in the spiral groove therein; means to electrically connect the ends of said resistor wire to an adjacent metallic end plug; a covering of epoxy material encapsulating said wire wound epoxy core, said covering having intimate bonding contact with the epoxy material of said core to form a unitary encapsulated resistor unit; and, electrical contact means extending outwardly from each of said metallic end plates to provide connections to electrical contacts.
 2. An electrical resistor unit according to claim 1 wherein said metallic end plugs are each formed with a circumferential groove, said plugs being positioned so as to be molded in said core in a manner that the epoxy material of said core completely fills the circumferential grooves of said end plates; said resistor wire is in the form of wire coil, with each turn being spaced equidistant apart; and, said epoxy covering being inTimately dispersed through the turns of said resistor wire coil.
 3. An electrical resistor according to claim 2 wherein said end plugs are provided with means for effecting an intimate interlock between said epoxy material of said core and said end plugs; whereby rotation of said end plugs relative to said core is resisted by said interlock means.
 4. An electrical resistor unit according to claim 3 wherein said means for effecting an intimate interlock between said end plugs and said epoxy core is at least one blind opening offset from the axes of said end plugs, and formed in the face of said plugs that face inwardly towards each other; whereby the epoxy material when said core is cast will enter into said openings to effect an interlocking engagement between said epoxy material of said core and said metallic end plugs to resist any tendency of said metallic plugs to rotate relative to said core.
 5. An electrical resistor; a solid cylindrical body defining a core formed of an epoxy material and having a spiral groove extending from one end to the other formed in the cylindrical surface thereof; a metallic end plug molded integrally with said core in each axial end of said core; electrical contact studs extending axially outwardly from said metallic end plugs; a coiled resistor wire wound on said core in the spiral groove thereof and extending from one end of said core to the other; means electrically connecting each end of said resistor coil wire to an adjacent metallic end plug to provide electrical continuity from one metallic end plug to the other; an epoxy covering intimately bonded to the epoxy material of said core to form an encapsulated resistor assembly, said epoxy covering being dispersed through the coils of said resistor wire to completely enwrap the wire in the epoxy material; a porcelain tubular housing having an internal diameter larger than the external diameter of said encapsulated resistor assembly to receive said resistor assembly therein; metallic end closures engaged on each end of said porcelain housing, said end closures being sealed to the ends of said housing, said enclosures having electrical contact with an adjacent axially extending contact stud; and, sulfur hexafluoride insulation filling the void within said housing.
 6. An electrical resistor according to claim 5 wherein the void within said housing is filled with urethane of soft durometer.
 7. An electrical resistor according to claim 5 wherein the void within said housing is filled with an insulating oil. 